1. Field of the Invention
The present invention relates to oxide sintered compact for preparing a transparent conductive film formed as an electrode in a flat panel display or the like. It also relates to a transparent conductive film obtained by using said oxide sintered compact as a sputtering target and a method for preparing the same.
2. Description of the Related Art
An ITO (Indium Tin Oxide) film is characterized in low resistivity and high transmission factor, and can be microfabricated easily. Since these characteristics are superior in comparison to other transparent conductive films, an ITO film is being broadly used in various fields including for use as a display electrode in a flat panel display. The deposition method of the ITO film in today's industrial production process is mostly based on the so-called sputter deposition method of performing sputtering using an ITO sintered compact as the target since the ITO film can be uniformly formed on a large area with favorable productivity.
In a flat panel display manufacture process using an ITO transparent conductive film, the crystallinity of the ITO film immediately after the sputtering is amorphous, and, in many cases, microfabrication such as etching is performed with the ITO film in an amorphous state, and thermal annealing is subsequently performed to crystallize the ITO film. This is because an ITO amorphous film is advantageous in terms of productivity due to much higher etching rate than an ITO crystalline film, while the ITO crystal film is superior in low resistivity, and both advantages can thereby be enjoyed.
Although most part of the film obtained by sputtering the ITO target is amorphous, in many cases a part of the film becomes crystallized. The reason for this is that although most part of the film remains amorphous since the temperature will become no more than 150° C., which is approximate to the crystallization temperature of the ITO film, particles that come flying to the substrate by sputtering sometimes have a high energy level and the temperature of the film becomes so high as to exceed the crystallization temperature due to the transfer of energy after the particles arrive at the substrate, resulting in crystallization of a part of the film.
If a part of the ITO film becomes crystallized in this way, such crystallized portion will remain as so-called etching residue in the subsequent etching process, and cause problems such as a short circuit, since the etching rate of that part will be lower than that of an amorphous portion by roughly two orders of magnitude.
In light of the above, it is known that the addition of water (H2O) in addition to sputter gas such as argon in a chamber during sputtering is effective as a method of preventing the crystallization of the sputtered film and amorphizing the entire sputtered film (for example, refer to Non-Patent Document 1).
Nevertheless, the method of adding water when sputtering to obtain an amorphous film entails numerous problems. Foremost, in many cases particles are generated on the sputtered film. These particles have an adverse effect on the flatness and crystallinity of the sputtered film. Since particles will not be generated if water is not added, the problem of generation of particles is caused by adding water.
In addition, even if the water concentration in the sputtering chamber is initially adequate, the concentration will gradually fall below the adequate concentration, and a part of the sputtered film will become crystallized, since it gradually decreases during the lapse of the sputtering time.
On the other hand, however, if the concentration of the water to be added is increased in order to reliably obtain an amorphous sputtered film, the crystallization temperature for crystallizing the film in the subsequent annealing process will become extremely high, causing a problem that the resistivity of the obtained film will become extremely high.
In other words, if sputtering is performed with adding water to amorphize the entire sputtered film, it is necessary to constantly monitor and control the water concentration within the chamber, however, this is extremely difficult and requires considerable time and effort.
In order to overcome the foregoing problems, a transparent conductive material which is stably amorphous is partly used in substitute for an ITO film which readily becomes crystallized. For instance, it is known that an amorphous film can be obtained by sputtering a target having a composition of indium oxide and an added amount of zinc, but the sputtered films obtained thereby are very stably amorphous, and 500° C. or more is necessary to crystallize them.
Therefore, the advantage on the process by crystallizing the film in order to significantly lower the etching rate cannot be obtained, and thus the resistivity of the sputtered film obtained is approximately 0.5 mΩ cm, which is higher than the crystallized ITO film. Moreover, the visible light average transmission factor of this film is roughly 85%, and is inferior to that of ITO film.
As prior art disclosing that amorphous films can be obtained without adding water and/or zinc, the following are referred to.
Patent Document 1 teaches “A transparent conductive film predominantly comprising of the oxide of In, furthermore containing Ge, or Ge and Sn can be amorphous, and thereby is easy to be etched and excellent in workability.” (Paragraph 0015). This is because that “under certain film forming conditions, adding Ge is effective for amorphizing In2O3 film and said addition does not have any bad influence upon the electric resistivity and transmission factor.” (Paragraph 0021). Such film forming conditions are that “the film forming temperature is adjusted to 100-300° C., and 2-12 atomic percent of Ge is added based on the total amount of Ge and In, and the oxygen partial pressure is kept at 0.02 mTorr or more when the film is formed” (Paragraph 0029). It is also described that “During the formation of the film, if the film forming temperature is below 100° C., and the amount of Ge added is less than 2 atomic percent, reduction of the electric resistivity due to carrier electron release by Ge is not sufficient, and therefore the electric resistivity is more than 0.01 Ω cm” (Paragraph 0030).
However, the Patent Document 1 describes only about obtaining an amorphous film, and thus crystallization of the amorphous films obtained is not considered at all.
Patent Document 2 discloses “the transparent conductive thin film containing indium oxide as a main ingredient and further containing at least one selected from W, Si, and Ge stably provides films with excellent surface smoothness and amorphous structure. The amorphous film is crystallized through heating to obtain a low resistivity of 9×10−4Ω cm or less while kiiping the surface smoothness, and furthermore a high visible light transmission factor is obtained (Paragraph 0024). The Patent Document 1 indicates that when one or more selected from W, Si, and Ge is(are) added as a first additive element to indium oxide or indium tin oxide, the amorphousness and surface smoothness of the films may be realized. The content of the first additive elements is adjusted to 0.2-15 atomic percent based on the total amount of In and the first additive elements. Further, it is described that the amorphous transparent conductive thin films are crystallized while keeping its excellent surface smoothness by heating treatment over the crystallization temperature and thereby the specific resistivity may be reduced.
Patent Document 2 discloses that the amorphous film is prepared and then crystallized. However, it only discloses an example where W is added and does not disclose any other examples where any one of the other elements is solely added. Therefore, it is not clear whether other elements such as Ge may accomplish the same effect as that of W. Further, in Examples, the heat treatment temperature for crystallization is as high as 300-400° C.
In addition, there is prior art for improving the properties by adding various elements into the sputtering target.
Patent Document 3 discloses a target containing an insulating oxide in addition to indium oxide or indium tin oxide, in order to provide an indium oxide system sputtering target for the highly resistant transparent conductive film with the resistivity of about 0.8-10×10−3Ω cm. As one example of an insulating oxide, manganese oxide is mentioned. However, Patent Document 3 does not describe any sputtering targets for obtaining a low resistant conductive film.
Patent Document 4 discloses a target adding Mn to indium oxide or indium tin oxide since Mn containing in the sintered compact made of indium oxide and tin oxide will enable an extremely high sintered density. It is described that the content of Mn is adjusted to 5 to 5000 ppm Mn in the ITO sintered body finally obtained. The content of Mn is preferably 10˜5000 ppm. In Examples, it was added 500 ppm at most.
Patent Document 5 discloses that resistivity of a transparent conductive film reduces by adding manganese oxide into indium oxide. According to the document, when 2-15 mol % of manganese oxide is added, the film has preferably resistivity of less than 2×10−4Ω cm.
Patent Document 6 describes a transparent conductive film which is mainly comprised of indium oxide and tin oxide, and includes an oxide of at least one metal selected from the group consisting of magnesium and nickel. In this way, the film becomes fine, increasing electron mobility to about 1.5×10 cm2·s−1·V−1. Further, Document 1 also discloses that resistance to humidity and ultraviolet rays can be improved by optimizing the ratio of magnesium or nickel. Document 1 teaches that a formulating proportion of magnesium compound or nickel compound to indium compound in terms of the formula represented by M/(M+In) (M is nickel or magnesium, and In is indium) is preferably 0.05 or less.
Patent Document 7 discloses that specific resistance of a transparent conductive film is reduced by adding nickel oxide to indium oxide. The amount of nickel oxide to be added is preferably defined as 2-25 mol % to achieve the specific resistance of 2×10−4Ω cm or less.
Patent Document 8 discloses a film comprised of indium oxide and further containing trivalent cation, an example thereof being aluminum. Patent Document 8 teaches that a transparent conductive film having lower resistance and improved etching workability is thus obtained.
[Prior Art Documents]
<Patent Document>
Patent Document 1: JP 3780100B
Patent Document 2: JP 2004-241296A
Patent Document 3: JP2003-105532A
Patent Document 4: JP 3496239B
Patent Document 5: JP H03-78907A
Patent Document 6: H07-161235A
Patent Document 7: H03-71510A
Patent Document 8: H08-199343A
<Non-Patent Document>
Non-Patent Document 1: Thin Solid Films 445 (2003) p 235-240